1. Field of the Invention
The present invention relates generally to an apparatus for controlling an automatic transmission of a motor vehicle, and more particularly to such an automatic transmission shift control apparatus for selecting an optimum operating position of the automatic transmission, according to shift boundary lines using an intake air quantity per revolution of the vehicle engine.
2. Discussion of the Prior Art
There is known a shift control apparatus for automatically controlling a vehicle automatic transmission, according to shift boundary lines which are represented by stored data maps and each of which represents a relationship between a throttle valve opening TA and the vehicle speed. The determination of whether the transmission is shifted down or up is effected by determining whether a point defined by the currently detected throttle valve opening and vehicle speed is positioned on one or the other side of the shift-down or shift-up boundary line which corresponds to the currently selected position of the transmission. Some modern vehicle engines are equipped with various devices such as a valve timing changing device, a swirl control valve device and an idling speed adjusting device. The operations of these devices may change the engine torque or torque characteristics. If the known shift control apparatus as indicated above is used for such engine, the transmission may be undesirably shifted at a varying drive torque, since the engine torque tends to fluctuate even with the same throttle valve opening. As a result, the transmission may be shifted frequently in a short period of time, or suffer from a considerable torque variation or shock upon shifting actions.
In view of the above drawback, there has been used an electronic shift control apparatus as disclosed in laid-open Publication No. 60-34563 of unexamined Japanese Patent Application. This shift control apparatus uses shift boundary lines each representing a relationship between the vehicle speed, and an intake air quantity Q/N per revolution of the vehicle engine, namely, an intake air quantity Q divided by an engine speed N. These parameters Q and N are detected by respective sensors. Since the intake air quantity Q/N per revolution of the engine (hereinafter referred to simply as "intake air quantity Q/N") accurately represents the engine torque, the transmission can be smoothly shifted, without busy or frequent shifting actions or a shifting shock.
However, the shift control apparatus using the intake air quantity Q/N and the vehicle speed for the shift boundary lines suffer from poor vehicle acceleration smoothness and response to the operation of the accelerator pedal, as felt by the vehicle driver. That is, the intake air quantity Q/N reaches the maximum value, and the engine torque reaches 90% or higher of the maximum value, when the throttle valve opening TA (hereinafter referred to as "throttle opening TA") is increased to a level of about 50-60%. Therefore, the driver's depression of the accelerator pedal to increase the throttle opening TA from that 50-60% level will not result in a considerable increase in the engine torque, whereby the vehicle acceleration as felt by the vehicle driver is bottomed at about 50-60% of the throttle opening.
The vehicle using the shift control apparatus as described above suffers from another problem, where the vehicle engine is provided with a turbocharger, a valve timing changing device or a swirl control valve device. More particularly, the intake air quantity Q/N which reflects the engine torque may increase, without depression of an accelerator pedal or without an increase in the opening of the throttle valve, due to an effect of the turbocharger, for example. As a result, the transmission may be commanded to be shifted down. This shift-down action is unexpected by the vehicle driver, since the driver is not requiring acceleration of the vehicle, i.e., is not depressing the accelerator pedal to increase the throttle opening. Namely, the vehicle driver depresses the accelerator pedal to increase the throttle opening to a given level, and the throttle opening is held constant with the accelerator pedal kept at the same position by the driver. The intake air quantity and the engine torque suddenly rise a considerable time after the depression of the accelerator pedal, due to the effect of the turbocharger. Consequently, the transmission may be shifted down unexpectedly to the vehicle driver, since the vehicle is not in the process of deceleration and the accelerator pedal is held at the same position.